changing how apport handles master bugs with private data

[Robert Collins]

https://bugs.launchpad.net/ubuntu/+source/apport/+bug/764414
has a summary in it of a chat pitti and I had on IRC about the
handling of master bugs with private data. What we currently do
consistently confuses users and prevents Launchpads duplicate bug
detection working properly (because the master bug cannot be seen).

The basic proposal is:
 - the master bug should be a public bug
 - there should be a link in the description for easy access by
developers to a private-master which has the confidential data
[crashdumps etc]
 - apport should do this automatically.

More details in the linked bug; we'd appreciate any
concerns/feedback/improvements folk can offer.

-Rob & Martin